These are all binaries needed by check-lld that need symlinks.
Also add gn/build/libs/xar (needed by llvm-objdump).
Also delete an incorrect part of the symlink description in //BUILD.gn (it used to be true before I made the symlink step write a stamp file; now it's no longer true).
Shouldn't this also symlink llvm-ar as ar if llvm_install_binutils_symlinks is set?